When it comes to affordable street performance engines, many enthusiasts are making serious horsepower by using small-blocks with some sort of centrifugal supercharger or by dumping in heavy doses of laughing gas. But we couldn't help but wonder if it were possible to assemble a big-inch Rat motor that shells out more than 500 ponies and 500 ft-lbs of torque, runs on low-octane pump gas, and most importantly, won't cost and arm and a leg to build? It seems that maybe some people are under a spell when it comes to thinking that a big-block can be assembled for a reasonable price tag and still offer big performance in return. Do those same folks think that big-blocks are a thing of the past? We hope not.
It's true that the new generation of crate motors offer a good starting point for potential horsepower in both big- and small-block configurations. And while you can spend a chunk of money trying to fill out your horsepower recipe with a 540-inch mega Rat, there's still a ton of usable street power to made from a less costly 454 starting point. With the help of a GM Performance Parts block, a stroker combination from Scat Engineering, and heads and induction components from Holley, that's exactly what we are going to demonstrate with this affordable build up.
We didn't start with a "crate motor" per se, but we did use a Gen 6 block as a basis for our streetable stroker and went from there. As typical as we are in comparison with every other "car guy" out there, we just couldn't stay happy with the stock displacement, so we went a little bit further now, to get a lot more out of it later.
We opted for a stroker crank from Scat Engineering, along with a set of I-beam rods. We punched the bore out by .030 and filled the holes with a lighter weight forged piston from SRP. The final displacement of this beast ended up at 490 inches. This still isn't as big as the most common crate motors like the 502s, but the power that will be produced will certainly be on par with those costly bigger engines.
As always the cylinder heads are really what makes the difference in power. Choosing a proper casting for a street motor can often be a task of it's own. Finding the correct head to flow good numbers from the start and make usable power is no easy challenge. One thing is for sure, a good oval port is really what this power plant needed.
Holley Performance Products had just the ticket, their large oval port aluminum head. This cylinder design provides immediate response from about 1,500 rpm, and continues to pull all the way to 6,000 rpm. This is the absolute best rpm range for an honest street motor. This will provide a ton of low-end power that will throw you in your seat the second you decide to stand on it.
A healthy street cam would also promote the low-end pull, while we looked to optimize the benefits of a late-model motor. Using the stock hydraulic-roller lifters and retaining plates is a wise choice, but at the same time using an aftermarket bumpstick is the best way to get the valve lift and duration numbers you're after. With that in mind we used GM Performance hydraulic-roller lifters with a Lunati camshaft that had a duration of 240-250 degrees at .050. The rest of the valve train was also from Lunati, like the roller rockers and pushrods.
We finished off the top with a Stealth dual-plane manifold from Weiand. Using a dual plane will also allow the motor to run at low rpm. Too many people think that a single plane is superior to a dual plane, but without a lot of compression or a high rpm motor, the single plane can cause a flat spot until the engine reaches a higher rpm, kind of like a stumble sensation.
After careful assembly, we tried out a few different carburetors when the engine was on the dyno, all of them from Holley Performance Products. In this we had the option of saving a little gas with a vacuum secondary, or getting a little more power with a double-pumper. In the end it was easy to settle, the 850-cfm double-pumper performed awesome. It idled much nicer and was more responsive, and of course made more power overall.
To build and dyno this engine we took a short trip over to our friends at Vrbancic Brothers Racing in Ontario, California. Both George and Bob have decades worth of experience building and racing Rat motors. And with our goal to put together a relatively inexpensive big-inch big-block, their expertise was invaluable. The short story here is using quality components that are matched together and being careful during the machining and assembly processes yielded us a stout motor that is sure to set us back into our seat cushions. With parts from Holley, SRP, and Milodon, coupled with Wayne's Auto in Riverside doing the machining steps and the Vrbancics putting her all together, we were sure that we'd be getting good numbers when we hit the dyno and reliability when it hits the streets. Check it out, and we're sure you'll agree.